The planographic printing can be carried out by using any one of a number of different types of printing plates. For instance, an imaged forming layer made of photosensitive resin is coated over a substrate, and the desired image portions are formed in the image forming layer by photographically selectively exposing the image forming layer with the aid of an original film placed over the surface of the image forming layer to thereby cure the image portions, and removing the portions of the image forming layer which were not cured. According to another conventional plate making process, an image forming layer made of photosensitive resin is coated over a substrate, and the desired image portions are formed in the image forming layer by selectively radiating an infrared laser beam onto the image forming layer so as to define and selectively cure the image portions on the image forming layer, and removing the portions of the image forming layer which were not cured. According to yet another plate making process, a foaming layer including small particles of thermally foaming material and light absorbing material is interposed between a substrate and an image forming layer, and a desired image is formed in the image forming layer by selectively heating the light absorbing material after a desired pattern and removing the image forming layer by the abrasive action of the thermally foaming material.
It is also known to melt a hot melt ink composition which contains photochemical material and is in solid form at room temperature, expel the molten ink composition from a nozzle in the form of liquid droplets so as to form the desired image on an aluminum plate having an anodized surface layer, and cure the ink composition by the radiation of light (see Japanese patent laid open publication No. 11-256085). According to another method, imaging ink containing a lipophilic component is sprayed from an ink jet recording head so as to form a desired image (see Japanese patent laid open publication No. 2001-219527). According to yet another method, a desired image is formed on a substrate given with a hydrophilic surface by spraying hydrophobic ink containing a light curing component onto the substrate from an ink jet recording head, and the entire surface is exposed to light so that the ink may be cured on the surface of the substrate (see Japanese patent laid open publication No. 4-69244).
According to such known methods of preparing planographic printing plates using an ink jet recording head, as no photographic developing step is required for the removal of the portions of the photosensitive resin which did not cure, the problems associated with the photographic process such as the costs and labor that are required in maintaining the developer and disposing the used developer can be avoided. Also, as no expensive equipment such as a high output infrared laser is required, the associated problems such as the high cost of the laser can be avoided. Also, the blank printing plates for such methods are highly simple and economical.
In a conventional printing plate having a photosensitive resin layer (regular PS plates and PS plates for dry CTP), the surface of an aluminum base plate may be subjected to an anodic oxidation for the purpose of improving the wear resistance of the printing plate and the photosensitive resin layer may be formed on the anodized surface. However, when a photosensitive layer is to be formed, it is necessary to seal the anodized layer by using hydrophilic material and thereby prevent the photosensitive material from seeping into the small pits of the anodized layer. Otherwise, the photosensitive material would seep into the small pits of the anodized layer, and get lodged therein so that the photosensitive material that has not cured and is required to be removed would fail to be removed cleanly off the printing plate. This is obviously undesirable.
In case of the plate making process using an ink jet recording head for spraying a hot melt ink composition proposed in Japanese patent laid open publication No. 11-256085, as there is a need to melt the ink composition which is in solid form at room temperature, a heating arrangement is necessary to keep the feeding unit and ink jet recording head warm, and this adds to the complexity of the system. Also, because the hot melt ink composition is so small in volume, the ink composition rapidly solidifies as soon as it reaches the printing plate having a relatively large heat capacity, and the bonding force between the printing plate and ink composition is undesirably weak. Furthermore, the surface of the conventional printing plate is sealed for the purpose of making the entire surface hydrophilic, and the ink composition is not able to penetrate into the surface layer of the printing plate. This also prevents the bonding force between the printing plate and ink composition from increasing to a desired degree.
In case of the plate making process proposed in Japanese patent laid open publication No. 2001-219527, the image portions which may consist of dots or lines tend to lack in thickness so that the produced printing plate may not be suited to make a large number of prints.
In case of the plate making process proposed in Japanese patent laid open publication No.4-69244, it is advantageous to be able to use common substrate material such as conventional PS plates, but the imaging resin deposited on the surface of the substrate tends to spread out so much that it is difficult to achieve a plate making process at a high resolution. Furthermore, the bonding force between the imaging resin and substrate is limited, and the produced printing plate is therefore not suited for making a large number of prints.
The conventional PS plate is produced by forming an anodized film over the coarsened surface of an aluminum base plate, making the surface hydrophilic and applying a layer of photosensitive resin material over the hydrophilic surface. The surface of the aluminum base plate is made both coarse and hydrophilic typically by sand blasting, electrolytic polishing, anodic oxidation and dipping into soda silicate (also for sealing the pits that may be present on the surface of the substrate). The PS plate is not provided with a porous layer that may help a suitable amount of the imaging resin to be retained on the surface because such a porous layer would prevent the photosensitive material from being etched away during the developing process. In Japanese patent laid open publication 2002-67521, it is discussed that the deep grooves produced as a surface coarsening process may cause small patches of the photosensitive material to be left on the surface of the printing plate.
In short, none of the conventional printing plates mentioned above can provide all of the desired attributes of a printing plate such as printing suitability (satisfactory deposition of ink on the image portions and resistance to soiling of the non-image portions), wear resistance (capability to make a large number of prints), a long shelf life and a high image resolution.
During the course of investigating the problems associated with the prior art, the inventors have discovered that the surface property of the printing plate is highly important in achieving the desired attributes of a printing plate. In particular, it was discovered that providing a certain porosity to the surface of the printing plate allows the desired attributes of a printing plate to be achieved.